This invention relates to an externally ignited fourcycle internal combustion engine in which the fuel-air mixture is formed outside of at least one cylinder. The present invention is concerned more particularly with such an internal combustion engine in which the inlet and outlet valves are arranged on one side of the cylinder. This engine is known as a side-valve or L-head engine. The combustion chamber of each cylinder in this engine is a volume surrounding the valve discs. It is designed to allow a substantially single vortex movement of the charge whereby the imaginary central axis of the swirl is substantially parallel to the cylinder axis. The height of the swirl chamber is typically less than its maximum diameter. A guide channel, originating approximately above the center of the piston head surface collects gases emerging from the cylinder volume when the piston approaches its top dead center position and directs them in a substantially tangential direction to the swirl chamber, thereby generating an essentially single swirl motion of the compressed charge during and toward the end of the compression stroke of the piston.